1. Field of the Invention
The present invention relates to an output shaft of a power unit integrally having an internal combustion engine and a transmission.
2. Description of Background Art
In the power unit, rotation of a crankshaft of the internal combustion engine is transmitted to the output shaft after having performed shift transmission by the transmission, and rotation of the output shaft projected from a unit case is transmitted to a drive wheel to rotate the same.
Since a high load is applied to the output shaft, a large output shaft manufactured by forging is normally used for securing strength (for example, JP-A-2003-139124.
The output shaft disclosed in JP-A-2003-139124 is a solid shaft manufactured entirely by forging and hence has a heavy weight.
However, by an ingenuity to increase the diameter of the output shaft only at borne portions, positioning is facilitated and slight reduction of the weight is achieved.
However, in order to reduce the diameter of portions other than the large-diameter borne portion of the output shaft, there is a limit in strength, and the reduction of the weight is only a small extent. In addition, a manufacturing device for integrally forming the entire output shaft by forging is also upsized, so that the facility cost is high.
In view of such problems, it is an object of the present invention to provide an output shaft of a power unit which may be manufactured by a small-sized manufacturing device, and may be reduced in weight while maintaining the strength.